Demos: 2C-07 Curve Balls

A V-shaped launcher is used to throw a large Styrofoam ball. The sandpaper provides the friction necessary to set the ball spinning rapidly. Depending on which way the ball is launched, it can be made to curve dramatically either right or left, and even upward, illustrating the Magnus force. Because of its light weight, the ball can be thrown into the audience.

Directions: Place the ball in the trough of the launcher. To make the ball curve to your right, swing the launcher horizontally to your left. Do this rapidly to get sufficient spin and launch speed. To get the ball to curve left, swing the launcher to your right. To get the ball to curve upward, place yourself in front of an aisle (for safety purposes) bring the launcher rapidly downward.

Suggestions for Presentation: Discuss the principle behind curve balls, using the Bernoulli effect. (The ball is deflected by the net force arising from the pressure difference on each side of the spinning ball, a force properly called the Magnus force.) Explain how the curve ball launcher works, then ask if the ball spins in, say, a counterclockwise direction, which way will it curve? After demonstrating an upward curve, explain that ordinary baseballs do not curve upward, i.e. there is no rise ball. The ball is too heavy. This will evoke arguments from baseball players, but film studies have been done with professional pitchers and no evidence has been seen for a ball rising. The likely explanation is that batters are accustomed to seeing the ball travel naturally downward in its flight. If it doesnt fall as quickly because of the backspin, it will give the appearance of having risen.

Applications: Curving baseballs. See also 2C - 12 and the Magnus effect on spinning cylinders.

Note: From a practical standpoint, a curving ball is likely more affected by a wake-deflection force than the Magnus force. For a detailed discussion, see Bloomfield, HOW THINGS WORK: The Physics of Everyday Life, John Wiley, 1997, pp. 187-189.